The construction of a capacitor from cells providing for a method of accumulating electric energy in an electric double layer on the surface of contact of an electrode and with an electrolyte therein is disclosed in many patents including U.S. Pat. No. 3,536,963. In this patent, the electrodes are comprised of particles of activated carbon having a great surface area and are impregnated with an electrolyte of either aqueous and/or non-aqueous nature. To collect a charge on the external side of the electrodes, current collectors are provided. The current collectors are made of a conducting thin-sheet of material that is impermeable to the electrolyte and is inert to it. The electrodes are held to their original shape by means of elastic gaskets around the outside perimeter. The elastic gaskets seal the electrodes from the environmental effects and further provide the electrical insulation from the side current collectors around the perimeter of the capacitor.
It should be understood that while the capacitors of the above patent are low voltage systems which possess a large capacitance created from the electric double layer, they are nevertheless limited by the decomposition voltage of the electrolyte. Therefore to get an increased operating voltage, it has become known in the art to stack these electric double layer capacitors or cells in a stack which has each cell connected in a series. The number of cells in the stack thus being determined by the required operating voltage needed and the operating limits of the collective cells. To obtain the current from these stacks, these stacks are provided with wire current lead outs and are mounted in a compartment type case of a dielectric material like polymethylmethacrilate with holes provided for said current lead outs to deliver the current wherever it is desired.
The problems with these type constructions is that the cells having a liquid electrolyte leak and form electrolyte jumpers between the adjacent cells which results in an increased leakage of current and the formation of gases subsequent of electrolysis of the electrolyte.
Further, these cells when placed in the stack are compressed initially, but over their life there is a decrease in the initial compression of the capacitor cells which increase the contact resistance factor in the cells and the overall stack.
Other patents like the electric double layer capacitor in Japanese lay open application 58-140110 attempt to solve these problems by forming a stack of capacitor cells with an electric double layer cell and current collector plates with current lead outs which are stacked and then compressed by a given force. In this formation process, the inner layer of the dielectric case closely embraces the stack of capacitor cells and the connected current collector plates, which factor prevents electrolyte short circuits between the cells and simultaneously makes for maintaining a compressive force necessary for providing the electrical contacts in the stack. Thus, the second dielectric layer of the case forms an external shell about the stack but still did not solve the compression decay life problem associated with compressed stacks of double electric layer capacitors.
The prior art has attempted in other prior art solutions to solve the compression decay problems by making electric double layer capacitors of a relatively small area as in up to 10 cm.sup.2 and using a low operating voltage by using a small number of capacitor cells in the stack which stored little energy as in up to 100 Joules and having a low discharge pulse power of up to 100 Watts, but with only limited success. Further, the prior art has also had little luck in the formation of large energy capacitors that have up to 100 cm.sup.2 of surface area and a great many of such capacitor cells as in 10 to 100 in the stack without the formation of cracks and loss of efficiency. These failures are in some cases caused by the large lateral surface over which changes in temperature have great effects. The mechanical effects of expansion can cause the dielectric case to crack and leak electrolyte which reduces the service life and operating safety of the device. Also the insulating cases are not really designed to maintain a great compression force of the stack in such large capacitors which can reach several tons of compression force. Using the current prior art cases with in the capacitors with these large forces makes the capacitor subject to loss of compression and accordingly increase in internal resistance of the capacitor.